Conventionally, in chroma key processing using an editing device at a broadcasting station or the like, picture synthesis is carried out by inserting a background picture into a foreground picture.
Specifically, at a broadcasting station or the like, for example, an announcer is imaged with a screen of blue color as a background (so-called blue back), thereby generating an image signal of a foreground picture. Also, a desired object to be allocated to the background of the foreground picture is imaged, thereby generating an image signal of a background picture.
In an editing device or the like, in response to the operation by an operator, a color to be extracted from the foreground picture is preset by setting a reference signal level with respect to color-difference signals. In the case where picture synthesis is to be carried out by using a soft chroma key, in the editing device, lower limit and upper limit signal levels (that is, threshold values) are set with reference to a center color of the color to be extracted.
The editing device generates a key signal by sequentially comparing the threshold values with the image signal of the foreground picture. Specifically, with respect to the foreground picture generated with the background of blue color screen, the editing device generates a key signal by setting the upper limit and lower limit threshold values so that a value 0 is obtained in the background part while a value 1 is obtained with colors other than the blue color of the background. With respect to other bluish colors, a key signal is generated within a range of the value 1 to the value 0 corresponding to the upper limit and lower limit threshold values so that a value corresponding to the degree of blueness is obtained.
Thus, the conventional editing device generates a key signal by sequentially discriminating the color of the foreground picture, using a two-dimensional color space expressed by color-difference signals (that is, a color space with UV signals as coordinate axes). The editing device synthesizes the foreground picture and the background picture with reference to the key signal thus generated, so as to generate a synthetic picture such that the background picture is inserted in the background of the foreground picture.
Meanwhile, in the conventional editing device, it is difficult to discriminate a portion of a high luminance level and a portion of a low luminance level with the same color. Therefore, with respect to a synthetic picture generated by the conventional editing device, there arises a problem such that the contour of the inserted background picture is displayed in an unnatural manner, thus lowering the definition in comparison with the case where no chroma key processing is carried out.
As a method for solving this problem, it may be considered to express and process each pixel of the foreground picture on a three-dimensional color space. In this method, however, the range of key signal setting must be adjusted on the three-dimensional color space instead of the conventional two-dimensional color space, and therefore the operation to adjust chroma key processing might be complicated.